Technological development and increased demand for mobile equipment have led to a rapid increase in the demand for secondary batteries as energy sources. Among these secondary batteries, lithium secondary batteries having high energy density and driving voltage, long lifespan and low self-discharge are commercially available and widely used.
In addition, increased interest in environmental issues has brought about a great deal of research associated with electric vehicles (EVs) and hybrid electric vehicles (HEVs) as substitutes for vehicles, such as gasoline vehicles and diesel vehicles, using fossil fuels which are major causes of air pollution. Nickel metal hydride (Ni-MH) secondary batteries are generally used as power sources of electric vehicles (EVs), hybrid electric vehicles (HEVs) and the like. However, a great deal of study associated with use of lithium secondary batteries, high energy density high discharge voltage and power stability is currently underway and some are commercially available.
In particular, lithium secondary batteries used for electric vehicles should have high energy density, exert high power within a short time and be used for 10 years or longer under harsh conditions, thus requiring considerably superior stability and long lifespan, as compared to conventional small lithium secondary batteries. In addition, secondary batteries used for electric vehicles (EVs), hybrid electric vehicles (HEVs) and the like require rate characteristics and power characteristics according to driving conditions of vehicles.
Conventional lithium secondary batteries generally utilize a lithium cobalt composite oxide having a layered structure for a cathode and a graphite-based material for an anode. However, such a lithium cobalt composite oxide is disadvantageously unsuitable for electric vehicles in terms of presence of extremely expensive cobalt as a main element and low safety. Accordingly, lithium manganese composite oxides having a spinel structure that comprise cheap and highly stable manganese are suitable for cathodes of lithium ion batteries for electric vehicles.
However, in case of lithium manganese composite oxides, manganese is eluted into an electrolyte during charge and discharge at high temperatures and high currents, causing deterioration in battery characteristics. Accordingly, there is a need for a solution to prevent this phenomenon. Also, lithium manganese composite oxides disadvantageously have smaller capacity per unit weight than conventional lithium cobalt composite oxides or lithium nickel composite oxides, thus having a limitation of increase in capacity per weight. Design of batteries to overcome this limitation is required so that lithium manganese composite oxides can be commercially applied to power sources for electric vehicles.
In order to solve these disadvantages, materials such as Li(NixMnyCozO2) (x+y+z=1) are used. In order to secure structural stability of such a layered-structure cathode active material, many researchers have studied cathode active materials with a layered structure containing Li2MnO3.
The cathode active materials with a layered structure containing Li2MnO3 are characterized in that Li is contained in a general transition metal layer made of LiMO2 (M: transition metal) and they have super lattice peaks caused by the Li2MnO3 structure. Such a material contains a great amount of Mn, thus being advantageously considerably cheap and exhibiting considerably high capacity and superior stability at a high voltage. The material has a broad voltage area of 4.4 to 4.6V. After activation occurs in the broad region, capacity increases. This increase in capacity is known to be caused by deintercalation of Li from the transition metal layer due to generation of oxygen, but opinions associated with the cause are still controversial.
Clearly, after the activation domain, structural variation is serious and electrical properties are thus deteriorated. The reason for this is known that structural variation causes conversion from a layered structure into a spinel structure and thus makes contact between domains loose. For these reasons, practical application of this substance to batteries is impossible at present.
In order to solve these problems, in the related art, a method in which particles of the active material are coated after synthesis, has been attempted, but this method disadvantageously causes an increase in preparation cost. Furthermore, as this method uses a post-treatment manner and does not substantially contribute to variation and improvement of inner structure, most structural variation is caused by formation of crystallinity at a high temperature of the synthesis process.